The present invention relates to a heater and more particularly to a heater employing a positive temperature coefficient (PTC) body.
Generally, the above described type of heater is employed in a compressor, used in, e.g., refrigerator or air-conditioning device, for maintaining the lubricant contained in the compressor housing above a predetermined temperature. In conventional refrigeration components, a refrigerant, such as those sold under the trademark "Freon" by E. I. du Pont de Nemours & Co., may, in liquid form, migrate from the condenser into the compressor lubricant. Upon start-up of the compressor, the sudden reduction in crankcase pressure may cause the refrigerant to boil, thus causing the lubricant to form with consequent loss of lubrication to other mechanical parts of the compressor. It has been conventional to employ a crankcase heater to maintain the compressor crankcase at a temperature above that of the rest of the refrigeration system so as to prevent the migration of refrigerant into the crankcase lubricant.
Formerly, fixed constant-resistance heaters were used for heating the crankcase. These heaters, however, were not self-regulating and thus required further temperature controls to limit the heat output of the heater so as to prevent damage to the lubricating oil. These constant-resistance heaters and their associated temperature controls were complicated and expensive.
Then, in order to eliminate such disadvantages, a heater made of ceramic material having a positive temperature coefficient (PTC) of resistivity has been proposed. Such a heater has a relatively low resistance at usual ambient temperatures, but after initial energization by a source of electrical power will self-heat and increase their temperature and resistance. Heat will be generated and the resistance will increase rapidly above a threshold or anomaly temperature until the heat generated balances the heat dissipated at which time the temperature and resistance stabilize with the resistance many times the initial value. Thus, these heaters are self-regulating at a temperature that will not exceed a safe value.
One prior art device employing the PTC element is shown in FIG. 1 includes potting compounds 1 to electrically insulate the PTC heater 2 from the heater case 3 to provide increased heat transfer from the heater to the case and to locate the heater within the case. However, it has been found that certain potting compounds, such as epoxy resin materials or the gases emitted therefrom, deleteriously affect the PTC heater when it is operated at high temperature.
In view of this, an electric heater having a barrier that separates the PTC body and the potting compounds is proposed and is disclosed in detail in U.S. Pat. No. 3,940,591 to Ting, issued on Feb. 24, 1976. However, according to this patent, no consideration is given to the efficiency of heat transmission from the outer surface of the PTC body to the outer surface of the casing.